In disk storage devices, data is stored on tracks concentric with an axis of rotation of the disk. To access a block of data (for reading or writing) a reading and writing head mounted on a suspension arm is positioned in the region of the desired track.
The density of data which can be stored in these devices is limited, amongst other things, by the resolution which can be achieved in the positioning of the suspension arm. A limitation is imposed, for example, by the fact that a fairly large force is required to overcome the static friction of the head. Once in motion, the suspension arm can then be stopped only after it has travelled a minimum distance. Moreover, the deformability of the suspension arm introduces oscillations which do not allow the reading and writing head to be positioned precisely and kept in the desired position.
The deformability of the suspension arm also limits the speed of its response to a positioning request, slowing down the operation of the storage device as a whole. For example, a minimum waiting time is required in order to damp the oscillations induced in the suspension arm. Moreover, the movement of the suspension arm has to be sufficiently gradual to prevent mechanical resonance phenomena.
The density of data which can be stored and the operating speed can be increased with the use of a suspension arm having an articulated structure as described, for example, in U.S. Pat. No. 5,521,778. As described therein, a main actuator positions the suspension arm roughly and a second actuator brings about fine and rapid positioning of a tip of the suspension arm on which the reading and writing head is mounted.
A structure which enables the tip of the suspension arm to be deflected is used to correct errors in the positioning of the head on the track, as described in "Piezoelectric Microactuator Compensating for Off-Track Errors in Magnetic Disk Drives," T. Imamura, etal., Adv. Info. Storage Syst., Vol.5, 1993 ISSN 1053-184X, pp. 119-126 ("Imamura"). In Imamura, the suspension arm includes an easily deformable portion which forms a hinge about which the tip of the suspension arm can pivot. Two plates of piezoelectric material arranged parallel to the disk are provided on the two sides of the hinge at a distance such as to define a suitable lever. When one piezoelectric plate lengthens and the other shortens the deformable portion is bent slightly and the tip of the suspension arm consequently pivots about the hinge.
A disadvantage of this solution is that the deformable structure thus produced weakens the suspension arm excessively. This introduces oscillations which reduce the resolution and the speed of the positioning of the suspension arm. Moreover, the greater deformability of the suspension arm increases the risk of accidental contact between the head and the disk with consequent damage to the storage device.
The object of the present invention is to avoid the aforementioned drawbacks.